Diseases of the nervous system are collectively the leading cause of human disability, as measured by the global burden of disease. Even those major diseases of the nervous system for which treatments have been approved by health authorities, including psychiatric diseases such as Schizophrenia, neurological diseases such as Alzheimer's Disease, and neurodevelopmental disorders, such as Attention Deficit and Hyperactivity Disorder, are poorly managed because approved treatments have limited efficacy and serious side effects, leaving a significant burden of unmet medical need. In addition, there are many major and rare nervous system disorders for which no treatments are approved, such as the neurodevelopmental disorders of the Autism Spectrum, and many intellectual disability disorders, and which are therefore associated with profound unmet medical need.
The N-methyl-D-aspartate-(NMDA) subtype of ligand-gated ion channel receptors are a diverse family of glutamate receptors widely accepted to mediate synaptic transmission, key mechanisms of synaptic plasticity, and dynamic neuronal network connectivity required for normal nervous system development and function.
The NMDA receptor is composed of four protein subunits, two GluN1 subunits and two GluN2 subunits. The GluN1 subunit is derived from a single gene (GRIN1), is ubiquitously expressed throughout the nervous system, and is common to all NMDA receptors. Four different GluN2 subunits, GluN2A-D, are derived from separate genes (GRIN2A-D) that are differentially expressed in different regions of the nervous system and by distinct populations of neurons within a particular region. A GluN3 subunit has also been identified, but its function is less well understood. Furthermore, individual neurons may express more than one GluN2 subunit and individual NMDA receptors expressed by such neurons may contain two of the same GluN2 subunits (for example, 2GluN2B subunits) or two different subunits (one GluN2A and one GluN2B subunit). In addition, all NMDA receptor subunits are expressed as diverse mRNA splice variants. Thus, native nervous system NMDA receptors are highly diverse in their composition.
The study of the molecular basis of NMDA receptor function continues to be an area of importance. As glutamate is the major excitatory neurotransmitter, dysfunction of glutamate neurotransmission and NMDA receptor-dependent mechanisms of synaptic transmission, plasticity, and neuronal network connectivity are broadly implicated in diseases of the nervous system. Accordingly, compounds that are capable of modulating NMDA receptors may be useful for the treatment of nervous system disorders and diseases, for example, schizophrenia, Alzheimer's disease, attention deficit and hyperactivity disorder, and autism.